The prior art is replete with oral compositions containing zinc salts for various beneficial effects believed to be imparted by the zinc ion, for example, reduction of calculus formation or inhibition of offensive mouth odor due to fermentation and putrefaction occurring in the oral cavity. However, despite the heretofore known use of zinc compounds in dental compositions, their use has not been without certain drawbacks. For example, when such soluble zinc compounds have been employed, it has not been possible to satisfactorily incorporate a soluble ionic fluoride salt, such as an alkali metal, ammonium, stannous and the like fluorides, for anti-caries activity in the compositions due to the inherent chemical incompatibility therebetween in the presence of water. This is generally evidenced by the precipitation of insoluble fluoride with a resultant decrease in the amount of efficacious fluoride ion available in the composition.
In U.S. Pat. No. 2,527,686, for example, a mouthwash is disclosed containing soluble zinc chloride, soluble ionic fluoride and, in addition, formaldehyde and two ferments, papain and malt. However, the zinc/fluoride combination is not stable in such a formulation as evidenced by undesirable cloudiness and precipitation of insoluble by-products.
Attempts have thus been made either to use insoluble or sparingly soluble zinc salts or to utilize zinc which has been chemically modified in some manner, for example, by interaction with other adjuvants in order to decrease its potential for forming an insoluble byproduct in the presence of ionic fluoride, such as by complexation with a zinc complexing agent, or by addition of a solvating agent to effect solvation of the zinc salt, or the like. For example, in U.S. Pat. No. 4,138,477, oral compositions are disclosed containing a zinc-polymer complex formed by the reaction of a zinc salt and an anionic polymer; in U.S. Pat. Nos. 4,289,754 and 4,325,939, an alkali metal or ammonium zinc citrate complex is described; and in U.S. Pat. No. 4,289,755, entitled "Stable Mouthwash Compositions Containing Zinc and Fluoride Compounds", the solvation of zinc citrate by the addition of citric acid is described.
Either or both of the zinc and fluoride salts may also be physically modified in order to lessen the problem of zinc/fluoride incompatibility for example, by use of well known sustained release, microencapsulation or the like physical forms which allow permeation of the particular salt from protective particles into the oral composition over a period of time, thereby reducing the rate of undesirable interaction between the two salts. For example, in U.S. Pat. No. 4,220,552, microencapsulation of sodium fluoride by a particular pretreated form of a lower alkyl cellulose is disclosed for use in dental formulations.
It is therefore highly desirable to provide a stable oral composition containing a water soluble zinc salt capable of preventing and controlling mouth odor but which does not present any substantial chemical incompatibility when admixed with an ionic fluoride salt in the presence of water. It is also desirable that said zinc salt and said fluoride salt be utilized as such without the need for either or both being modified by chemical or physical means.